The present invention relates to the art of nanotechnology, and in particular, to carbon nanotube technology, its function and structure.
A carbon nanotube is a single graphene sheet in the form of a seamless cylinder. The ends of a nanotube typically have hemispherical caps. The typical diameter of a nanotube ranges from about 1 nm to 10 nm. The length of a nanotube potentially can be millions of times greater than its diameter.
Carbon nanotubes are comprised of shells of sp2-hybridized carbon atoms forming a hexagonal network that is itself arranged helically within the cylinder. Basically, helicity is the arrangement of the carbon hexagonal rings with respect to a defined axis of a tube. (M. S. Dresselhaus et al “Science of Fullerenes and Carbon Nanotubes” (Academic Press, New York, 1996)).
Carbon nanotubes are grown by combining a source of carbon with a catalytic nanostructured material such as iron or cobalt at elevated temperatures. At such temperatures, the catalyst has a high solubility for carbon. The carbon links up to form graphene and wraps around the catalyst to form a cylinder. Subsequent growth occurs from the further addition of carbon.
Since their discovery in the early 1990s, carbon nanotubes have been the focus of intense study due to their very desirable and unique combination of physical properties. They are chemically inert, thermally stable, highly strong, lightweight, flexible and electrically conductive. In fact, carbon nanotubes may potentially be stiffer and stronger than any other known material.
Carbon nanotubes are currently being proposed for numerous applications, such as, for example, catalyst supports in heterogeneous catalysis, high strength engineering fibers, sensory devices and molecular wires for the next generation of electronics devices.
There has been particularly intense study of the electrical properties of nanotubes, and their potential applications in electronics. Metallic carbon nanotubes have conductivities and current densities that meet or exceed the best metals; and semiconducting carbon nanotubes have mobilities and transconductance that meet or exceed the best semiconductors.
The physical properties of carbon nanotubes are structure-dependent. For example, depending on the diameter and helicity of a nanotube, the tube can be either metallic or semi-conducting. Also, a single structural defect in a hexagonal ring can change a metallic nanotube to a semiconducting nanotube. Current methods for producing nanotubes results in a mixture of tubes with diverse diameters, helicities and structural defects. Thus, a mixture of metallic and semi-conducting nanotubes are produced.
A method of rationally modifying the physical properties of nanotubes is to chemically functionalize them with an intrinsically configurable moiety. One such moiety is the family of semiconductor nanocrystals or quantum dots, such as CdSe and CdTe, which exhibit strongly size-dependent optical and electrical properties (Murray et al., Annu. Rev. Mater. Sci. 30:546 (2000)). Nanocrystals are selected according to the physical properties desired to be imparted to a nanotube for a particular end use application. The products of such functionalization methods have been referred to as nanotube-nanocrystal heterostructures (Azamian et al., Chem. Commun. 4:366–7 (2002); Ravindran et al., Nano Lett. 3:447 (2003); and Haremza et al., Nano Lett. 2:1253 (2002)).
There are significant shortcomings of current methods by which to modify the physical properties of nanotubes. For example, there has been poor control over the surface coverage and degree of clustering of nanocrystals on nanotubes. The surface coverage and clustering directly affect the physical properties of the resulting nanotube. Thus, current methods do not allow for precise control of the physical properties of nanotubes.
In addition to modifying the physical properties of nanotubes, the ability to controllably assemble nanotubes into various designed architectures is essential to building complex, functional nanotube devices. That is, nanotubes need to be arranged into well-defined supramolecular configurations in order to design nanoscale integrated systems. Such integrated systems are required for applications such as high-efficiency computing; high-density data storage media; light harvesting in photovoltaic cells; lightweight, high-strength textiles; microelectromechanical devices; supersensitive sensors; and drug delivery agents.
Current methods of producing heterostructures are not amenable to controllably arranging nanotubes into well-defined supramolecular configurations. That is, the resulting heterostructures are primarily unorganized, discrete one-dimensional “ball-and-stick” structures. Such structures are difficult to organize into orderly assemblies which are necessary for nanoscale integrated systems.
Also, the ability to make electrical contacts between individual nanotubes within a nanotube integrated system is essential. However, current methods do not allow for reliable control over the nature of the contact between nanotubes.
Thus, the shortcomings of current methods of modifying the properties of nanotubes and of utilizing nanotubes in integrated systems present major obstacles to actualizing the utility of carbon nanotubes for end use applications.
Accordingly, there remains a need for a method of producing carbon nanotubes with particular physical properties that are necessary for various end use applications, in particular for applications pertaining to integrated systems.